1. Field of the Invention
The present invention relates, in general, to radio frequency (RF) power amplifiers. It is particularly useful in the type of RF power amplifiers used in wireless telephones.
2. Description of the Related Art
Code Division Multiple Access (CDMA) modulation techniques are one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA), and amplitude modulation (AM) modulation schemes such as amplitude companded single sideband (ACSSB) are known, CDMA has significant advantages over these other techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307 entitled "Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters," assigned to the assignee of the present invention, the disclosure thereof incorporated by reference.
In wireless telephone systems, a plurality of wireless telephones communicate with one or more base stations within "cells." In Code Division Multiple Access (CDMA) wireless telephone systems, in particular, wherein all portable users within a cell are sharing the same band of frequencies, but using different codes to modulate their respective signals, each wireless telephone's transmitter signal becomes noise to all of the others. Keeping the overall noise level down tends to help all users. This is particularly true for IS-95, the CDMA standard. Wireless telephones are called upon to transmit at a wide range of power levels depending upon conditions existing at the time of transmission. A mobile CDMA phone is required to transmit at maximum power relatively infrequently. For the greater part of the time it is better to transmit at relatively low power levels. When low power levels are required it is desirable that the current-consuming high-power stages be turned off, and bypassed. Typically, CDMA wireless phones operate at a large number of discrete power levels; the levels are selected in response to changing transmission conditions.
Since wireless telephones operate on battery power, it is also desirable that their transmitters operate as efficiently as possible to conserve power and extend battery life. Ideally for CDMA systems, such as those governed by the IS-95 standard, power amplifier stages should be capable of efficient, linear operation over their required dynamic range. However, the state of the art has not yet come close to the ideal and many wireless telephones now in use have poor power management. During low power transmissions, power is wasted by cascaded amplifier stages that are not needed. Consequently, attempts have been made to bypass unused stages.
Such attempts have required the high power amplifier's output to pass through a switch. For example, see U.S. Pat. No. 5,661,434, issued Aug. 26, 1997 to Brozovich et. al. entitled "High Efficiency Multiple Power Level Amplifier Circuit," the subject matter of which is incorporated herein by reference. Brozovich et al utilizes three single-pole single-throw switches to bypass stages of a multi-stage amplifier. A bypass switch is always in series with the amplifier stage to be bypassed. However, it is difficult to utilize switches in series with high power amplifier stages. Generally, switches have a loss associated with them. Forcing the output of high power amplifier stages through a switch can cause an unacceptable loss. Furthermore, the types of switches that must be used for high power are both large and expensive, causing unacceptable design constraints. Consequently, there is a need for High Efficiency Power Amplifier (HEPA) circuits for CDMA wireless telephone applications that can bypass amplifier stages when the stages are not necessary.
A High Efficiency Power Amplifier (HEPA) may not be efficient in the traditional sense. Amplifier efficiency is most often specified at maximum output power. The efficiency of class A amplifiers and class AB amplifiers tends to zero as the output power is reduced. High efficiency in the "HEPA" sense refers to the average current drawn by the amplifier in a specified operating environment. As discussed in copending U.S. patent application Ser. No. 08/579,169 entitled "Efficient Parallel-Stage Power Amplifier," filed Dec. 25, 1995 and assigned to the assignee of the present invention, and hereby incorporated by reference, and additionally copending U.S. patent application Ser. No. 08/767,124 entitled "Efficient Parallel-Stage Power Amplifier," filed Dec. 9, 1996 also assigned to the assignee of the present invention, also hereby incorporated by reference, the utilization of these types of power control techniques requires that the portable unit transmitter be capable of linear operation over a relatively wide dynamic range.
The operating environment of a CDMA wireless telephone is typically urban or suburban. Both environments have common salient features: the probability of operating at high output power (such as more than 15 dBm for IS-95), or at low output power (such as less than -15 dBm for IS-95) is small. It is most probable that the power amplifier output power is in the neighborhood of 0 dBm. Power amplifiers that are efficient at high output power (28 dBm for IS-95) are usually not efficient at 0 dBm.
The cause of low efficiency in Class A and class AB amplifiers at low output power is their idle or quiescent current. The level of quiescent current is chosen to satisfy the design requirements at maximum output power, but is typically still drawn from the supply when the power amplifier is not transmitting at maximum power. Since the power amplifier is seldom at maximum power the quiescent current is wasted most of the time.
A typical power amplifier consists of several serial stages. Each stage is usually larger, and more powerful than the previous one. Most of the quiescent current is drawn by latter high power stages, which are not required for the low output power levels at which the phone is often called upon to transmit. It follows that bypassing the high power stages when they are not required can make a significant saving in current.
Of course, the individual stages of a power amplifier add more than power, they add gain. Fortunately, the decrease in gain that accompanies bypassed stages is a bonus, as it decreases the dynamic range required by previous circuitry.